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Permitted 802.11b transmitter power

Posted on 2004-08-03
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Last Modified: 2013-11-09
Hi, I wonder if anyone can help ?
I am currently using a Cisco Aironet 1200 AP with the standard rubber duck aerials.
As I understand it, these dipoles have a gain of 2.2dBi. In the UK we are limited by ETSI regulations in this band to a maximum EIRP of 100mW (20 dBm), which means the AP tx power has to be set to 50mW (17 dBm), which gives an EIRP of 19.2 dBm = 83mW
I need to extend the coverage slightly and was wondering about using a higher gain aerial on the AP. However since I would then have to lower the tx power to 30mW to avoid going over the 20dBm limit, it might not give any improvement. Has anyone any suggestions, and my question is: since the actual received power right next to the AP (measured using a Fluke Waverunner) is only about -30 dBm, why is the permitted EIRP set so low as it seems that most of the tx power is not radiated anyway ?
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Question by:chris_smailes
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6 Comments
 
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by:rfgkev
ID: 11714131
you cannot extend the coverage much by doing this, the only gain would be a boost to the recieved signal strength, so unless you can do the same on the clients you will not get a much extended range. However the 2.2 dbi antennae usually give a 'Doughnut shaped' signal, which gives good coverage around and above the antennae, take a look at something like a 6dbi omni, they give a flatter signal but usually better range. Again, it also depends on the client aswell as to the range you can get. Some manufacturers do produce range extending antennae for their client adapters.

Unknown why the EIRP for our region (ETSI) is so low, especially compared to mobile phones (>3 W). have had this conversation with a number of manufacturers of wireless equipment but still have no satisfactory answer, best one was "They probably picked that number out of a hat".

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by:chris_smailes
ID: 11733726
Thanks for that information; it tallies with the way I was thinking, but I still don't understand why the measured radio signal power within a few mm of the AP aerial is so low.
Even taking into account the SWR of the aerial, which would give an 11% loss in signal, it is clear that nothing like the +18.5 dBm I calculated is being radiated. It seems that the radio signal strength from the aerial is a quantity of microwatts rather than the expected milliwatts.
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by:rfgkev
ID: 11733790
-30 is what you would expect being right next to the antennae, although you are sending nearly 20db to transmit, conducting it to the air and passing through the air causes a massive initial drop in power, having worked mainly on external point - point links we used to work on the fact that a signal would drop roughly 100 - 110db in the first Km (known as Free Space Loss)

The actual calculation used is

Lp= 92.45 + 20Log10F + 20Log10d

Lp= Path loss in decibels
F= frequency in GHz
dB= decibels
d= Distance in kilometres
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rfgkev earned 500 total points
ID: 11733805
Take a look at http://www.solwise.co.uk/los.htm it will give u a bit more info
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Author Comment

by:chris_smailes
ID: 11735763
Thanks - I've been investigating and found out similar information - which brings me back to the point that the ETSI EIRP limit is too low but we'll just have to live with it and plan around it, especially, as you point out, that most client cards can't actually transmit at that level.
Thanks again.
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Expert Comment

by:rfgkev
ID: 11735985
most of the client cards i've worked with only transmit at around 15db so as you said, the 20db limit isn't really much of an issue until you start playing with external point to point links.
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